1. Field of the Invention
The present invention relates to power scaling of erbium lasers, which are based on glass hosts and are primarily pumped at about 975 nm, and particularly with lasers having high dopant concentrations in a low phonon glass.
2. Invention Disclosure Statement
Erbium-lasers in the middle infrared spectral region are useful for medical and measurement applications, because they operate around 2.7 xcexcm. Prior art diode pumped erbium-lasers, however are limited to output power around 1 W for an erbium-doped crystal laser such as Er:YAG or Er:YSGG (D.-W. Chen, C. L. Fincher, T. S. Rose, F. L. Vernon and R. A. Fields, Optics Letters 24, 385 (1999)). For many desired applications, higher output powers are needed. This further scaling, needed especially for various medical/surgical applications, has been hindered by thermal lensing, degradation in output beam quality and frequent rod fracture problems (S. D. Jackson, T. A. King and M. Pollnau, Optics Letters 24, 1133 (1999)], which are common for doped crystal lasers.
Recently researchers have been able to generate 1.7 W from a fiber laser based on erbium co-doped with praseodymium. The dopant concentrations were restricted to rather low values in a ZBLAN fluoride glass matrix. Modest efficiencies were reported; 17.3% slope efficiency at 2.71 xcexcm for the diode pumped Er3+xe2x80x94Pr3+ ZBLAN laser. (Jackson et al.). The need to use a sensitizer-activator pair complicates the system, the economics and the commercialization of the product. The best result was obtained with pumping power of 22.4 W with 790 nm light and about a 45% coupling efficiency for the light incident on the fiber laser""s end surface.
Other crystal based lasers have been tried somewhat successfully in power output, 1 W output and efficiencies of 20-25%. In one prior art method, populating the upper laser-level directly at 970 nm has been used to pump Er3+-doped crystals, with doping concentrations of 10-15 molar % (T. Jensen, A. Diening, G. Huber and B. H. T. Chai Optics Letters 21, 585 (1996)). An up-conversion process depopulates the lower laser level while xe2x80x9crecyclingxe2x80x9d pump energy partially populates the multiplet again. This pumping scheme seems to be efficient, but requires high doping concentrations. The disadvantages of a crystal substrate are still present. A remaining problem is however, that typically concentrations above 1 molar % have not been achieved in ZBLAN host materials.
Some fluoride glasses were made in bulk with higher molar % of Er and were tested by the inventors (T. Sandrock, A. Diening and G. Huber, Optics Letters 24, 382 (1999)). Using laser diodes with 972 nm output and a fluoride glass with 8 molar % of Er, they found lasing output in the 2.7-2.8 xcexcm region. The efficiency was low, 10%, but the threshold power was very promising. A major remaining problem is a desire to gain the efficiency benefits of a fiber laser configuration, but realizing that drawing a fluoride glass with high dopant concentrations can lead to destabilization of the glass with crystallite formation during the draw process.
The present invention solves the main problems in the prior art by extending this work in bulk glasses to provide scalable erbium fiber lasers operating in the 2.8 xcexcm region.
It is an object of the present invention to provide a erbium ion fiber laser whose output is scalable above 1 w, without requiring co-dopants.
It is another object of the present invention to provide erbium fiber lasers operating in the vicinity of 3 xcexcm and having output greater than 1 w.
It is yet another object of the present invention to provide an erbium laser which is efficiently pumped by sources operating at around 975 nm because of higher slope efficiencies and reduced bleaching of the ground state at this pumping wavelength.
Briefly stated the present invention provides a diode pumped fiber laser that utilizes highly erbium-doped fluoride glass to produce 1 W and higher outputs at about 3 xcexcm. High dopant concentrations in fluoride glass make it possible to efficiently pump the laser around 975 nm which in turn leads to higher slope efficiencies for the laser. Furthermore, by using this pumping wavelength, saturation of laser output due to bleaching of the ground state is overcome. The fluoride glass composition is a modified version of the standard ZBLAN composition. This altered composition achieves high glass quality, enables fiber drawing, and avoids the problem of crystallization.